1. Field of the Invention
The present invention relates to an improved process and apparatus to reduce or eliminate species of fungi and algae from commercial greenhouse water using ultraviolet radiation.
2. Description of the Prior Art
Such fungi as Pythium, Phythophthora, and Fusarium spp in water seriously hamper the production of greenhouse vegetables grown hydroponically by the Nutrient Film Technique (NFT).
Nutrient Film culture is a viable soilless method for intensive crop production. The plants are grown in shallow channels containing a flowing nutrient solution which is continuously recirculated with some make up water added as the plants transpire. An NFT installation is a closed system with a volume of liquid maintained constant by a float valve which controls the supply of make up water and nutrient levels of the systems.
Advantages of the system include the efficient use of water and the elimination of expensive time-consuming processes associated with soil management.
One of the greatest dangers is that a disease causing organism will enter one channel and will be spread throughout the system by the recirculating solution and the whole crop will rapidly be wiped out. Pathogenes and pests may contaminate the system through water supplies. Growers are often faced with root rot, tobacco mosaic virus and wilt organisms, particularly Fusarium, Pythium, and Verticillium spp. Once systems are contaminated with pathogenes, the organisms are rapidly transmitted in the recirculating nutrient solution, causing severe crop injury and loss. The lack of pesticides registered in Canada for use in hydroponic systems makes control of pathogenes a persistent problem.
In addition to plant pathogenes, algae growth is commonly seen in NFT systems. Algae may be found i coating holding tanks and troughs and may cause blockage in flow pipes and tubes. Once algae populations have become established, a great deal of hand labour is required to maintain effective levels of sanitation. Algae may also compete directly with the crop for oxygen and dissolved nutrients, thus requiring more frequent change of nutrient solution in the NFT system.
The primary mode of action in an ultraviolet fungicidal process is to damage the nucleus of the irradiated organism. The UV rays act to cleave nucleic acids resulting in damage to chromosomal components. This process is called lethal sectoring. Damaged cells that retain the ability to form clones are characterized by slow growth rates, increased radiosensitivity and a high degree of variability in the size and morphology of the cells and macro colonies. UV damage is also demonstrated by a partial loss of progeny in the next few generations.
The physiological process of inactivation, however, is not well understood and is complicated by the ability of many organisms to repair UV damage inflicted to their nucleic acid.
In spite of this, it is well known that exposure of microorganisms to UV light with wave length near 2537.ANG. results in biochemical changes lethal to some organisms. The destruction of other bacteria, spores or viruses suspended in liquids very often gives survival curves of nearly logarithmic shape A 100% kill is rarely achieved.
Prior art fungicidal processes, used in England, include prophylactic substances which will prevent growth of disease causing organisms in the recirculating solution without adversely affecting crop production. Such substances are added to the solution and maintained at optimum concentration However, they present problems.
Adding the suggested 20 ppm of active etridiazole, as has been used in the United Kingdom, might be an example. Although such a concentration might possibly have no harmful effect on a tolerant crop, such as tomatoes, it might, under NFT conditions, have an adverse effect on more sensitive crops such as cucumbers. Etridiazole at a concentration of 20 ppm does not kill fungi; it merely inhibits their growth. It is, therefore, necessary to add etridiazole regularly because the substance does break down. There is mounting cost of application.
A different problem has been presented with an alternative prior art process. Aerial spray methods of disease control may be used in hydroponic greenhouses in England. These methods are similar to those applied to a conventional soil-grown crop. However, should some of the foliar spray enter the NFT channel, there is an undesirable effect of the solution around the roots.
In an NFT installation, the recirculating solution and the supply of make up solution can flow through a single pipe. It is convenient, therefore, to place in such a pipe, between the circulating pump and the inlet to the first NFT channel or in catchment tanks, an ultraviolet fungicidal unit. Because UV is a physical rather than chemical agent, it can be applied to an NFT system without producing toxic chemical residuals.
Preliminary trials suggest that the use of UV units affects the chelated iron in the solution Iron salts are essential nutrients in Nutrient Film culture Experimentation shows that some forms of chelated iron are more affected than others Mineral deposits out of the liquid form a coating on the surface of the UV lamp.
Prior art ultraviolet sterilizer units, such as manufactured by Trojan Environmental Products, are fitted with mechanical cleaners to remove precipitate from the bulb surface.
Another prior art sterilizer is described in Canadian Patent No. 1,062,437 by James H. Lewis. This disposable liquid sterilizer unit (comprising a jacket and tube) was clearly designed for house or cottage owners who require drinking water. The assembly also includes an activated carbon filter. The ABS jacket unit is factory sealed to the UV bulb and suffers from the disadvantage of reduced efficiency when the light source and jacket walls become coated with residue from water. In time, the unit must, therefore, be discarded.
Several forms of prior art commercially available UV sterilizers include an air space chamber between the UV bulb surface and the liquid envelope. Air drawn along the lamp can become ozone rich and brought in contact with cold untreated water. This process prevents precipitate build-up on the bulb. Many prior art commercially available UV sterilizers have been constructed with shielding devices to prevent cold water contacting the tube. Cold water on the surface of irradiated bulbs tends to decrease the life of underwater bulbs. Using cold contaminated water, with underwater UV bulbs, the kill rate drops. As disclosed in Canadian Patent No. 1,054,331, issued May 15, 1979, ultraviolet lamps operate at maximum efficiency on warm untreated water.
As the bulb becomes used and old, the voltage drop across the bulb changes. The wave length of light delivered also changes. Sterilizers currently available are fitted with complex switch devices, electrical current limiting resisters and light emitting diodes which indicate insufficient current to sterilize water.